Make sure that PhpStorm use 64-bit Java and not bundled one (which is 32-bit). You an check that in Help | About.

Generally speaking: with 64-bit Java you do not have to change that value as it works a bit differently compared to 32-bit one.

1) PhpStorm comes with bundled x86 Java and it is the first one that it tries; only then it looks for other Java installations -- check PhpStorm.bat for details (what environment variables and in which order). By defining one of those environment variables (which will point to your 64-bit Java installation) you can tell PhpStorm to use instead of bundled one.

2) PhpStorm v8 (currently in EAP stage) comes with 64-bit specific files: you should use PhpStorm64.exe and not PhpStorm.exe (same for .vmoptions file -- it should be PhpStorm64.exe.vmoptions).

I'm not sure how PhpStorm v7 works with 64-bit Java -- have never tried it this way myself.